Beet Armyworm

Beet armyworms lay their eggs in distinctive cottony masses on leaf surfaces. Newly hatched beet armyworms are small green worms that often feed in groups. Older beet armyworms vary in color, but usually have many fine, wavy, light-colored stripes down the back and a broader stripe down each side. The body appears smooth and hairless. The adult moth is mottled gray and brown with a wingspan of about 1 inch. The life cycle takes about a month in warm weather, and there are three to five generations a year.

These worms feed in the crown of the plant and can severely stunt or kill seedlings. The potential for damage is reduced between thinning and head formation. Once heads form, beet armyworms may cause serious damage by boring in from the bottom of the head which means that damage cannot be seen from above. In the San Joaquin Valley, fall populations are much more damaging than spring populations. In the central coast, beet armyworm damage starts to appear in late summer. In the low desert valleys, beet armyworm starts as a stand establishment pest during September through October and is a pest on developing lettuce heads in November and December.

Many natural enemies attack beet armyworms. Among them, parasitoids are most common and they include the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Viral disease (NPV or nucleopolyhedrovirus) also kill significant numbers.

Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders.

Use biological and cultural controls and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad on organically certified crops. Note that spinosad is detrimental to beneficial syrphid flies and parasitoids.

Start monitoring before seedlings emerge by checking for egg masses and young larvae in surrounding weeds. If numbers are high on weeds, watch carefully for larvae on seedlings.

• From germination to head formation, check at least 25 plants in each quadrant of a 40- to 80-acre field twice a week. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location. Fields smaller than 40 acres may require fewer samples.
• Check for egg masses and young larvae. Loopers, cabbageworms, armyworms, corn earworms, tobacco budworms, cutworms, and other caterpillars that feed on leaves and heads of lettuce can be assessed together, but species identification is important in choosing an insecticide.

Apply insecticide when the majority of eggs have hatched because most insecticides are less effective against insect eggs. Apply insecticide if you find:

• an average of one second or third instar larva per 10 young plants.
• more than one larva on every two older plants (after thinning and before heading).

Apply insecticide just before heading if this threshold is exceeded. For beet armyworm control, the best time to apply insecticide is at dawn or dusk (twilight hours).